WASHINGTON (CNN) --FBI Director Robert Mueller defended his agency's conduct in the war on terrorism Friday before a skeptical audience -- members of the American Civil Liberties Union.

He told the group that success will depend on whether terrorists can be beaten without trampling Americans' individual freedoms.

"We will be judged by future generations on how we react to this crisis," he said. "And by that, I mean not just whether we win the war on terrorism, because we will, but also whether, as we fight that war, we safeguard for our citizens the very liberties for which we are fighting."

The FBI director said the bureau won't "shy away from using every tool that Congress has given us to protect Americans against terrorism.

"In our free and exceptionally open society, there is no guarantee that there will never be another terrorist attack. And therefore we must thoroughly investigate every threat, whether at home or abroad, carefully -- carefully -- observing the constitutional rights of all."

Mueller spoke to 1,500 people attending the first nationwide gathering of ACLU members. The group billed the event as a "rallying cry" against what the ACLU sees as excesses by the Bush administration in trying to fight terrorism after the attacks of September 11, 2001.

Since then, the ACLU says, its membership rolls have increased by about a third, to 400,000. The group attributes the increase to growing public concern that individual freedoms are being threatened.

Introducing Mueller, ACLU Executive Director Anthony Romero expressed concern about the detention of immigrants, the expansion of law enforcement powers under the Patriot Act and the attempts of government to hold hearings in secret. He also said expanded use of technology to track Americans could create a "surveillance society."

"We know that the FBI has a difficult and necessary mandate to protect Americans from the next terrorist attack," Romero said. "We are here to deliver a short but powerful message -- to keep America safe and free."

Mueller conceded that in deciding how to balance the demands of law enforcement with the protection of civil liberties, "there are no easy answers." He said the FBI wrestles with those questions every day.

However, he said FBI agents are committed to working within constitutional guidelines and are extensively trained as to what they are allowed to do -- and what they aren't. He also noted that the FBI is up against "tenacious" terrorists who can easily adapt to changed circumstances.

"The ACLU seeks to prevent the tyranny of the majority from destroying our fundamental liberties," he said. "But in fighting terrorists, we seek to prevent the tyranny of the minority from destroying our fundamental way of life."

Mueller defended the Patriot Act, saying its primary benefit is "to tear down the walls between the criminal side and the intelligence side, not only within the FBI, but also between the FBI and the intelligence communities."

The USA Patriot Act was passed by Congress and signed into law by President Bush in October 2001. It includes provisions to allow authorization of "roving wiretaps," so that law enforcement officials can get court orders to wiretap any phone a suspected terrorist would use, along with greater subpoena power for e-mail records.

The act also allows the federal government to detain noncitizens suspected of terrorism for up to seven days without specific charges being filed.

Mueller also said it is important for the government to "maintain secrecy in certain areas, such as in our investigations, such as in the techniques we use in our investigations. And there are areas where secrecy is exceptionally valuable in order to deter attacks."

Following the speech, Mueller answered questions from ACLU members.

He was asked about a recent report from the Justice Department's inspector general that criticized the bureau's handling of detainees rounded up on immigration charges after September 11. The report found that many detainees who turned out to have no links to terrorism were held for long periods of time because the FBI was too slow in clearing them.

Mueller said the report needs to be placed in that context of what the FBI faced after September 11 -- not knowing how many other terrorists might be in the United States or "whether in the days, the months afterward, we would be susceptible to yet another attack."

But he also said the inspector general did "a very good job in pointing out areas where we could do things better."

Among those improvements, Mueller said, are establishing "clear and consistent" criteria for determining whether a detainee is a threat; increasing FBI manpower so that detainees are cleared more quickly; and improving communication between agencies within the Justice Department.

Asked whether he would support the creation of an ombudsman to monitor the FBI's handling of civil liberties issues, Mueller noted that his agency is already subject to significant oversight.

"I will tell you that we are not short on those who are looking over our shoulder," he said. "We have something like 89 reviews of the bureau ongoing at this time. And there is not a week that [goes] by that I am not someplace up on [Capitol] Hill when Congress is here."

Although the ACLU and Mueller have been at odds on a number of issues, his reception at Friday's forum was cordial. He received a hearty round of applause at the beginning when Romero thanked him for showing up.

"Actually, I'm here to recruit FBI agents," Mueller quipped.

The director and the ACLU were in agreement on one point -- both oppose creation of a new domestic intelligence agency, which would take over the FBI's intelligence role and leave it as a law enforcement agency.

In December 2002, an influential homeland security panel known as the Gilmore Commission recommended the creation of an agency to collect and analyze information about terrorist threats within the United States. Tom Ridge, head of the Homeland Security Department, has also said that he opposes such a step.

"The reality is that the two functions are synergistic in the fight against terrorism," Mueller said. "The combined responsibilities make the FBI uniquely situated to make strategic and tactical choices between our law enforcement options of arrest and incarceration and our intelligence options of surveillance and source development."